Metrology method and apparatus

ABSTRACT

A method comprising: evaluating a plurality of polarization characteristics associated with measurement of a metrology target of a substrate processed using a patterning process, against one or more measurement quality parameters; and selecting one or more polarization characteristics from the plurality of polarization characteristics based on one or more of the measurement quality parameters.

FIELD

The present disclosure relates to methods and apparatus for inspection (e.g., metrology) usable, for example, in the manufacture of devices by lithographic techniques and to methods of manufacturing devices using lithographic techniques.

BACKGROUND

A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.

Significant aspects to enabling a patterning process (i.e., a process of creating a device or other structure involving patterning (such as lithographic exposure or imprint), which may typically include one or more associated processing steps such as development of resist, etching, etc.) include developing the process itself, setting it up for monitoring and control and then actually monitoring and controlling the process itself. Assuming a configuration of the fundamentals of the patterning process, such as the patterning device pattern(s), the resist type(s), post-lithography process steps (such as the development, etch, etc.), it is desirable to setup the apparatus in the patterning process for transferring the pattern onto the substrates, develop one or more metrology targets to monitor the process, set up a metrology process to measure the metrology targets and then implement a process of monitoring and/or controlling the process based on measurements.

So, in a patterning process, it is desirable to determine (e.g., measure, simulate using one or more models that model one or more aspects of the patterning process, etc.) one or more parameters of interest, such as the critical dimension (CD) of a structure, the overlay error between successive layers (i.e., the undesired and unintentional misalignment of successive layers) formed in or on the substrate, etc.

It is desirable to determine such one or more parameters of interest for structures created by a patterning process and use them for design, control and/or monitoring relating to the patterning process, e.g., for process design, control and/or verification. The determined one or more parameters of interest of patterned structures can be used for patterning process design, correction and/or verification, defect detection or classification, yield estimation and/or process control.

Thus, in patterning processes, it is desirable frequently to make measurements of the structures created, e.g., for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes, which are often used to measure critical dimension (CD), and specialized tools to measure overlay, a measure of the accuracy of alignment of two layers in a device. Overlay may be described in terms of the degree of misalignment between the two layers, for example reference to a measured overlay of 1 nm may describe a situation where two layers are misaligned by 1 nm.

Various forms of inspection apparatus (e.g., metrology apparatus) have been developed for use in the lithographic field. These devices direct a beam of radiation onto a target and measure one or more properties of the redirected (e.g., scattered) radiation—e.g., intensity at a single angle of reflection as a function of wavelength; intensity at one or more wavelengths as a function of reflected angle; or polarization as a function of reflected angle—to obtain a “spectrum” from which a property of interest of the target can be determined. Determination of the property of interest may be performed by various techniques: e.g., reconstruction of the target by iterative approaches such as rigorous coupled wave analysis or finite element methods; library searches; and principal component analysis.

A further technique is involves having the zeroth order of diffraction (corresponding to a specular reflection) blocked, and only higher orders are processed. Examples of such metrology can be found in PCT patent application publication nos. WO 2009/078708 and WO 2009/106279, which are hereby incorporated by reference in their entirety. Further developments of the technique have been described in U.S. patent application publication nos. US 2011-0027704, US 2011-0043791 and US 2012-0242940, each of which is incorporated herein in its entirety. Such diffraction-based techniques are typically used to measure overlay. The targets for techniques can be smaller than the illumination spot and may be surrounded by product structures on a substrate. A target can comprise multiple periodic structures, which can be measured in one image. In a particular form of such a metrology technique, overlay measurement results are obtained by measuring a target twice under certain conditions, while either rotating the target or changing the illumination mode or imaging mode to obtain separately the −1st and the +1st diffraction order intensities. The intensity asymmetry, a comparison of these diffraction order intensities, for a given target provides a measurement of target asymmetry, that is asymmetry in the target. This asymmetry in the target can be used as an indicator of overlay error.

SUMMARY

In the example of overlay measurement, they rely on an assumption that overlay (i.e., overlay error and deliberate bias) is the only cause of target asymmetry in the target. Any other asymmetry in the target, such as structural asymmetry of features within the periodic structure in an upper layer, within the periodic structure in a lower layer overlaid by the periodic structure in the upper layer, or both, also causes an intensity asymmetry in the 1st (or other higher) orders. This intensity asymmetry attributable to such other asymmetry in the target, and which is not related to overlay (including an intentional bias), perturbs the overlay measurement, giving an inaccurate overlay measurement. Asymmetry in the lower or bottom periodic structure of a target is a common form of structural asymmetry. It may originate for example in substrate processing steps such as chemical-mechanical polishing (CMP), performed after the bottom periodic structure was originally formed. It is desirable to improve the detectability, accuracy and/or robustness of metrology measurements such as overlay measurements.

In an embodiment, there is provided a method comprising: evaluating a plurality of polarization characteristics associated with measurement of a metrology target of a substrate processed using a patterning process, against one or more measurement quality parameters; and selecting one or more polarization characteristics from the plurality of polarization characteristics based on one or more of the measurement quality parameters.

Further features and advantages, as well as the structure and operation of various embodiments, are described in detail below with reference to the accompanying drawings. It is noted that the invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described, by way of example only, with reference to the accompanying drawings in which:

FIG. 1 depicts an embodiment of a lithographic apparatus;

FIG. 2 depicts an embodiment of a lithographic cell or cluster;

FIG. 3 schematically depicts an example inspection apparatus and metrology technique;

FIG. 4 schematically depicts an example inspection apparatus;

FIG. 5 illustrates the relationship between an illumination spot of an inspection apparatus and a metrology target;

FIG. 6 schematically depicts a process of deriving a plurality of variables of interest based on measurement data;

FIG. 7A depicts a schematic diagram of an inspection apparatus (e.g., a dark field scatterometer in this case) configured to measure a target using a first pair of illumination apertures;

FIG. 7B schematically depicts a detail of a diffraction spectrum of a target periodic structure for a given direction of illumination;

FIG. 7C schematically depicts a second pair of illumination apertures providing further illumination modes in using the inspection apparatus of FIG. 7A for diffraction based overlay measurements;

FIG. 7D schematically depicts a third pair of illumination apertures combining the first and second pair of apertures;

FIG. 8 depicts a form of multiple periodic structure target and an outline of a measurement spot on a substrate;

FIG. 9 depicts an image of the target of FIG. 8 obtained in the inspection apparatus of FIG. 7A;

FIG. 10 is a flowchart showing steps of an overlay measurement method using the inspection apparatus of FIG. 3;

FIG. 11A, FIG. 11B and FIG. 11C respectively show schematic cross-sections of overlay periodic structures having different overlay values in the region of zero;

FIG. 11D is a schematic cross-section of an overlay periodic structure having structural asymmetry in a bottom periodic structure due to processing effects;

FIG. 12 is a schematic cross-section of an overlay periodic structure used for making measurements according to an embodiment of the invention;

FIG. 13 illustrates principles of overlay measurement in an ideal target, not subject to structural asymmetry;

FIG. 14 illustrates principles of overlay measurement in a non-ideal target, with correction of structural asymmetry as disclosed in embodiments herein;

FIG. 15 is a graph of diffraction efficiency for a target for measurement at various polarization angles for a single wavelength;

FIG. 16 is a graph of stack sensitivity for a target for measurement at various polarizations for a single wavelength;

FIG. 17 is a schematic diagram of part of a metrology apparatus according to an embodiment of the invention;

FIG. 18 is a graph of overlay sensitivity for a target for measurement at various phase differences between the two orthogonal polarizations of the illuminating light and wavelengths of illumination radiation;

FIG. 19 is a schematic diagram showing how overlay sensitivity can be increased according to an embodiment of the invention.

DETAILED DESCRIPTION

Before describing embodiments in detail, it is instructive to present an example environment in which embodiments may be implemented.

FIG. 1 schematically depicts a lithographic apparatus LA. The apparatus includes an illumination optical system (illuminator) IL configured to condition a radiation beam B (e.g., UV radiation or DUV radiation), a patterning device support or support structure (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device in accordance with certain parameters; a substrate table (e.g., a wafer table) WT constructed to hold a substrate (e.g., a resist coated wafer) W and connected to a second positioner PW configured to accurately position the substrate in accordance with certain parameters; and a projection optical system (e.g., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., including one or more dies) of the substrate W.

The illumination optical system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.

The patterning device support holds the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The patterning device support can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The patterning device support may be a frame or a table, for example, which may be fixed or movable as required. The patterning device support may ensure that the patterning device is at a desired position, for example with respect to the projection system. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate, for example if the pattern includes phase-shifting features or so called assist features. Generally, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam, which is reflected by the mirror matrix.

As here depicted, the apparatus is of a transmissive type (e.g., employing a transmissive mask). Alternatively, the apparatus may be of a reflective type (e.g., employing a programmable mirror array of a type as referred to above, or employing a reflective mask).

The lithographic apparatus may also be of a type wherein at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the projection system and the substrate. An immersion liquid may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems. The term “immersion” as used herein does not mean that a structure, such as a substrate, must be submerged in liquid, but rather only means that liquid is located between the projection system and the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from a radiation source SO. The source and the lithographic apparatus may be separate entities, for example when the source is an excimer laser. In such cases, the source is not considered to form part of the lithographic apparatus and the radiation beam is passed from the source SO to the illuminator IL with the aid of a beam delivery system BD including, for example, suitable directing mirrors and/or a beam expander. In other cases the source may be an integral part of the lithographic apparatus, for example when the source is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system.

The illuminator IL may include an adjuster AD for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL may include various other components, such as an integrator IN and a condenser CO. The illuminator may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross section.

The radiation beam B is incident on the patterning device (e.g., mask) MA, which is held on the patterning device support (e.g., mask table) MT, and is patterned by the patterning device. Having traversed the patterning device (e.g., mask) MA, the radiation beam B passes through the projection optical system PS, which focuses the beam onto a target portion C of the substrate W, thereby projecting an image of the pattern on the target portion C. With the aid of the second positioner PW and position sensor IF (e.g., an interferometric device, linear encoder, 2-D encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor (which is not explicitly depicted in FIG. 1) can be used to accurately position the patterning device (e.g., mask) MA with respect to the path of the radiation beam B, e.g., after mechanical retrieval from a mask library, or during a scan.

Patterning device (e.g., mask) MA and substrate W may be aligned using patterning device alignment marks M₁, M₂ and substrate alignment marks P₁, P₂. Although the substrate alignment marks as illustrated occupy dedicated target portions, they may be located in spaces between target portions (these are known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the patterning device (e.g., mask) MA, the patterning device alignment marks may be located between the dies. Small alignment markers may also be included within dies, in amongst the device features, in which case it is desirable that the markers be as small as possible and not require any different imaging or process conditions than adjacent features. The alignment system, which detects the alignment markers is described further below.

Lithographic apparatus LA in this example is of a so-called dual stage type which has two substrate tables WTa, WTb and two stations—an exposure station and a measurement station—between which the substrate tables can be exchanged. While one substrate on one substrate table is being exposed at the exposure station, another substrate can be loaded onto the other substrate table at the measurement station and various preparatory steps carried out. The preparatory steps may include mapping the surface control of the substrate using a level sensor LS and measuring the position of alignment markers on the substrate using an alignment sensor AS. This enables a substantial increase in the throughput of the apparatus.

The depicted apparatus can be used in a variety of modes, including for example a step mode or a scan mode. The construction and operation of lithographic apparatus is well known to those skilled in the art and need not be described further for an understanding of the embodiments of the present invention.

As shown in FIG. 2, the lithographic apparatus LA forms part of a lithographic system, referred to as a lithographic cell LC or a lithocell or cluster. The lithographic cell LC may also include apparatus to perform pre- and post-exposure processes on a substrate. Conventionally these include spin coaters SC to deposit resist layers, developers DE to develop exposed resist, chill plates CH and bake plates BK. A substrate handler, or robot, RO picks up substrates from input/output ports I/O1, I/O2, moves them between the different process apparatus and delivers then to the loading bay LB of the lithographic apparatus. These devices, which are often collectively referred to as the track, are under the control of a track control unit TCU which is itself controlled by the supervisory control system SCS, which also controls the lithographic apparatus via lithography control unit LACU. Thus, the different apparatus can be operated to maximize throughput and processing efficiency.

In order to design, monitor, control, etc. the patterning process (e.g., a device manufacturing process) that includes at least one patterning step (e.g., an optical lithography step), the patterned substrate can be inspected and one or more parameters of the patterned substrate are measured. The one or more parameters may include, for example, overlay between successive layers formed in or on the patterned substrate, critical dimension (CD) (e.g., critical linewidth) of, for example, features formed in or on the patterned substrate, focus or focus error of an optical lithography step, dose or dose error of an optical lithography step, optical aberrations of an optical lithography step, etc. This measurement may be performed on a target of the product substrate itself and/or on a dedicated metrology target provided on a substrate. There are various techniques for making measurements of the structures formed in the patterning process, including the use of a scanning electron microscope, image-based measurement or inspection tools and/or various specialized tools. A relatively fast and non-invasive form of specialized metrology and/or inspection tool is one in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered (diffracted/reflected) beam are measured. By comparing one or more properties of the beam before and after it has been scattered by the substrate, one or more properties of the substrate can be determined. This may be termed diffraction-based metrology or inspection.

FIG. 3 depicts an example inspection apparatus (e.g., a scatterometer). It comprises a broadband (white light) radiation projector 2 which projects radiation onto a substrate W. The redirected radiation is passed to a spectrometer detector 4, which measures a spectrum 10 (intensity as a function of wavelength) of the specular reflected radiation, as shown, e.g., in the graph in the lower left. From this data, the structure or profile giving rise to the detected spectrum may be reconstructed by processor PU, e.g. by Rigorous Coupled Wave Analysis and non-linear regression or by comparison with a library of simulated spectra as shown at the bottom right of FIG. 3. In general, for the reconstruction the general form of the structure is known and some variables are assumed from knowledge of the process by which the structure was made, leaving only a few variables of the structure to be determined from the measured data. Such an inspection apparatus may be configured as a normal-incidence inspection apparatus or an oblique-incidence inspection apparatus.

Another inspection apparatus that may be used is shown in FIG. 4. In this device, the radiation emitted by radiation source 2 is collimated using lens system 120 and transmitted through interference filter 130 and polarizer 170, reflected by partially reflecting surface 160 and is focused into a spot S on substrate W via an objective lens 150, which has a high numerical aperture (NA), desirably at least 0.9 or at least 0.95. An immersion inspection apparatus (using a relatively high refractive index fluid such as water) may even have a numerical aperture over 1.

As in the lithographic apparatus LA, one or more substrate tables may be provided to hold the substrate W during measurement operations. The substrate tables may be similar or identical in form to the substrate table WT of FIG. 1. In an example where the inspection apparatus is integrated with the lithographic apparatus, they may even be the same substrate table. Coarse and fine positioners may be provided to a second positioner PW configured to accurately position the substrate in relation to a measurement optical system. Various sensors and actuators are provided for example to acquire the position of a target of interest, and to bring it into position under the objective lens 150. Typically many measurements will be made on targets at different locations across the substrate W. The substrate support can be moved in X and Y directions to acquire different targets, and in the Z direction to obtain a desired location of the target relative to the focus of the optical system. It is convenient to think and describe operations as if the objective lens is being brought to different locations relative to the substrate, when, for example, in practice the optical system may remain substantially stationary (typically in the X and Y directions, but perhaps also in the Z direction) and only the substrate moves. Provided the relative position of the substrate and the optical system is correct, it does not matter in principle which one of those is moving in the real world, or if both are moving, or a combination of a part of the optical system is moving (e.g., in the Z and/or tilt direction) with the remainder of the optical system being stationary and the substrate is moving (e.g., in the X and Y directions, but also optionally in the Z and/or tilt direction).

The radiation redirected by the substrate W then passes through partially reflecting surface 160 into a detector 180 in order to have the spectrum detected. The detector 180 may be located at a back-projected focal plane 110 (i.e., at the focal length of the lens system 150) or the plane 110 may be re-imaged with auxiliary optics (not shown) onto the detector 180. The detector may be a two-dimensional detector so that a two-dimensional angular scatter spectrum of a substrate target 30 can be measured. The detector 180 may be, for example, an array of CCD or CMOS sensors, and may use an integration time of, for example, 40 milliseconds per frame.

A reference beam may be used, for example, to measure the intensity of the incident radiation. To do this, when the radiation beam is incident on the partially reflecting surface 160 part of it is transmitted through the partially reflecting surface 160 as a reference beam towards a reference mirror 140. The reference beam is then projected onto a different part of the same detector 180 or alternatively on to a different detector (not shown).

One or more interference filters 130 are available to select a wavelength of interest in the range of, say, 405-790 nm or even lower, such as 200-300 nm. The interference filter may be tunable rather than comprising a set of different filters. A grating could be used instead of an interference filter. An aperture stop or spatial light modulator (not shown) may be provided in the illumination path to control the range of angle of incidence of radiation on the target.

The detector 180 may measure the intensity of redirected radiation at a single wavelength (or narrow wavelength range), the intensity separately at multiple wavelengths or integrated over a wavelength range. Furthermore, the detector may separately measure the intensity of transverse magnetic- and transverse electric-polarized radiation and/or the phase difference between the transverse magnetic- and transverse electric-polarized radiation.

The target 30 on substrate W may be a 1-D grating, which is printed such that after development, the bars are formed of solid resist lines. The target 30 may be a 2-D grating, which is printed such that after development, the grating is formed of solid resist pillars or vias in the resist. The bars, pillars or vias may be etched into or on the substrate (e.g., into one or more layers on the substrate). The pattern (e.g., of bars, pillars or vias) is sensitive to change in processing in the patterning process (e.g., optical aberration in the lithographic projection apparatus (particularly the projection system PS), focus change, dose change, etc.) and will manifest in a variation in the printed grating. Accordingly, the measured data of the printed grating is used to reconstruct the grating. One or more parameters of the 1-D grating, such as line width and/or shape, or one or more parameters of the 2-D grating, such as pillar or via width or length or shape, may be input to the reconstruction process, performed by processor PU, from knowledge of the printing step and/or other inspection processes.

In addition to measurement of a parameter by reconstruction, diffraction-based metrology or inspection can be used in the measurement of asymmetry of features in product and/or resist patterns. A particular application of asymmetry measurement is for the measurement of overlay, for example, but other applications are also known. In this case, the target 30 typically comprises one set of periodic features superimposed on another. For example, asymmetry can be measured by comparing opposite parts of the diffraction spectrum from the target 30 (for example, comparing the −1st and +1^(st) orders in the diffraction spectrum of a periodic grating). The concepts of asymmetry measurement using the instrument of FIG. 3 or FIG. 4 are described, for example, in U.S. patent application publication US2006-066855, which is incorporated herein in its entirety by reference. Simply stated, while the positions of the diffraction orders in the diffraction spectrum of the target are determined only by the periodicity of the target, asymmetry in the diffraction spectrum is indicative of asymmetry in the individual features which make up the target. In the instrument of FIG. 4, where detector 180 may be an image sensor, such asymmetry in the diffraction orders appears directly as asymmetry in the pupil image recorded by detector 180. This asymmetry can be measured by digital image processing in unit PU, and calibrated against known values of overlay.

FIG. 5 illustrates a plan view of a typical target 30, and the extent of illumination spot S in the apparatus of FIG. 4. To obtain a diffraction spectrum that is free of interference from surrounding structures, the target 30, in an embodiment, is a periodic structure (e.g., grating) larger than the width (e.g., diameter) of the illumination spot S. The width of spot S may be smaller than the width and length of the target. The target in other words is ‘underfilled’ by the illumination, and the diffraction signal is essentially free from any signals from product features and the like outside the target itself. The illumination arrangement 2, 120, 130, 170 may be configured to provide illumination of a uniform intensity across a back focal plane of objective 150. Alternatively, by, e.g., including an aperture in the illumination path, illumination may be restricted to on axis or off axis directions.

FIG. 6 schematically depicts an example process of the determination of the value of one or more variables of interest of a target pattern 30′ based on measurement data obtained using metrology. Radiation detected by the detector 180 provides a measured radiation distribution 108 for target 30′.

For a given target 30′, a radiation distribution 208 can be computed/simulated from a parameterized model 206 using, for example, a numerical Maxwell solver 210. The parameterized model 206 shows example layers of various materials making up, and associated with, the target. The parameterized model 206 may include one or more of variables for the features and layers of the portion of the target under consideration, which may be varied and derived. As shown in FIG. 6, the one or more of the variables may include the thickness t of one or more layers, a width w (e.g., CD) of one or more features, a height h of one or more features, and/or a sidewall angle α of one or more features. Although not shown, the one or more of the variables may further include, but is not limited to, the refractive index (e.g., a real or complex refractive index, refractive index tensor, etc.) of one or more of the layers, the extinction coefficient of one or more layers, the absorption of one or more layers, resist loss during development, a footing of one or more features, and/or line edge roughness of one or more features. The initial values of the variables may be those expected for the target being measured. The measured radiation distribution 108 is then compared at 212 to the computed radiation distribution 208 to determine the difference between the two. If there is a difference, the values of one or more of the variables of the parameterized model 206 may be varied, a new computed radiation distribution 208 calculated and compared against the measured radiation distribution 108 until there is sufficient match between the measured radiation distribution 108 and the computed radiation distribution 208. At that point, the values of the variables of the parameterized model 206 provide a good or best match of the geometry of the actual target 30′. In an embodiment, there is sufficient match when a difference between the measured radiation distribution 108 and the computed radiation distribution 208 is within a tolerance threshold.

A further inspection apparatus suitable for use in embodiments is shown in FIG. 7A. A target T and diffracted rays of measurement radiation used to illuminate the target are illustrated in more detail in FIG. 7B. The inspection apparatus illustrated is of a type known as a dark field metrology apparatus. The inspection apparatus may be a stand-alone device or incorporated in either the lithographic apparatus LA, e.g., at the measurement station, or the lithographic cell LC. An optical axis, which has several branches throughout the apparatus, is represented by a dotted line O. In this apparatus, radiation emitted by source 11 (e.g., a xenon lamp) is directed onto substrate W via optical element 15 by an optical system comprising lenses 12, 14 and objective lens 16. These lenses are arranged in a double sequence of a 4F arrangement. A different lens arrangement can be used, provided that it, e.g., provides a substrate image onto a detector, and simultaneously allows for access of an intermediate pupil-plane for spatial-frequency filtering. Therefore, the angular range at which the radiation is incident on the substrate can be selected by defining a spatial intensity distribution in a plane that presents the spatial spectrum of the substrate plane, here referred to as a (conjugate) pupil plane. In particular, this can be done by inserting an aperture plate 13 of suitable form between lenses 12 and 14, in a plane which is a back-projected image of the objective lens pupil plane. In the example illustrated, aperture plate 13 has different forms, labeled 13N and 13S, allowing different illumination modes to be selected. The illumination system in the present examples forms an off-axis illumination mode. In the first illumination mode, aperture plate 13N provides off-axis radiation from a direction designated, for the sake of description only, as ‘north’. In a second illumination mode, aperture plate 13S is used to provide similar illumination, but from an opposite direction, labeled ‘south’. Other modes of illumination are possible by using different apertures. The rest of the pupil plane is desirably dark as any unnecessary radiation outside the desired illumination mode will interfere with the desired measurement signals.

As shown in FIG. 7B, target T is placed with substrate W normal to the optical axis O of objective lens 16. The substrate W may be supported by a support (not shown). A ray of measurement radiation I impinging on target T from an angle off the axis O gives rise to a zeroth order ray (solid line 0) and two first order rays (dot-chain line +1 and double dot-chain line −1). It should be remembered that with an overfilled small target, these rays are just one of many parallel rays covering the area of the substrate including metrology target T and other features. Since the aperture in plate 13 has a finite width (necessary to admit a useful quantity of radiation), the incident rays I will in fact occupy a range of angles, and the diffracted rays 0 and +1/−1 will be spread out somewhat. According to the point spread function of a small target, each order +1 and −1 will be further spread over a range of angles, not a single ideal ray as shown. Note that the periodic structure pitches of the targets and the illumination angles can be designed or adjusted so that the first order rays entering the objective lens are closely aligned with the central optical axis. The rays illustrated in FIGS. 7A and 7B are shown somewhat off axis, purely to enable them to be more easily distinguished in the diagram.

At least the 0 and +1^(st) orders diffracted by the target T on substrate W are collected by objective lens 16 and directed back through optical element 15. Returning to FIG. 7A, both the first and second illumination modes are illustrated, by designating diametrically opposite apertures labeled as north (N) and south (S). When the incident ray I of measurement radiation is from the north side of the optical axis, that is when the first illumination mode is applied using aperture plate 13N, the +1 diffracted rays, which are labeled +1(N), enter the objective lens 16. In contrast, when the second illumination mode is applied using aperture plate 13S the −1 diffracted rays (labeled −1(S)) are the ones which enter the lens 16.

A beam splitter 17 divides the diffracted beams into two measurement branches. In a first measurement branch, optical system 18 forms a diffraction spectrum (pupil plane image) of the target on first sensor 19 (e.g. a CCD or CMOS sensor) using the zeroth and first order diffractive beams. Each diffraction order hits a different point on the sensor, so that image processing can compare and contrast orders. The pupil plane image captured by sensor 19 can be used for focusing the inspection apparatus and/or normalizing intensity measurements of the first order beam. The pupil plane image can also be used for many measurement purposes such as reconstruction.

In the second measurement branch, optical system 20, 22 forms an image of the target T on sensor 23 (e.g. a CCD or CMOS sensor). In the second measurement branch, an aperture stop 21 is provided in a plane that is conjugate to the pupil-plane. Aperture stop 21 functions to block the zeroth order diffracted beam so that the image of the target formed on sensor 23 is formed only from the −1 or +1 first order beam. The images captured by sensors 19 and 23 are output to processor PU which processes the image, the function of which will depend on the particular type of measurements being performed. Note that the term ‘image’ is used here in a broad sense. An image of the periodic structure features as such will not be formed, if only one of the −1 and +1^(st) orders is present.

The particular forms of aperture plate 13 and field stop 21 shown in FIGS. 7A, 7C and 7D are purely examples. In an embodiment, on-axis illumination of the targets is used and an aperture stop with an off-axis aperture is used to pass substantially only one first order of diffracted radiation to the sensor. In yet other embodiments, 2nd, 3rd and higher order beams (not shown in FIG. 7A, 7B, 7C or 7D) can be used in measurements, instead of or in addition to the first order beams.

In order to make the measurement radiation adaptable to these different types of measurement, the aperture plate 13 may comprise a number of aperture patterns formed around a disc, which rotates to bring a desired pattern into place. Note that aperture plate 13N or 13S can only be used to measure periodic structures oriented in one direction (X or Y depending on the set-up). For measurement of an orthogonal periodic structure, rotation of the target through 90° and 270° might be implemented. Different aperture plates are shown in FIGS. 7C and 7D. The use of these, and numerous other variations and applications of the apparatus are described in the patent application publications mentioned above.

FIG. 8 depicts a (composite) target formed on a substrate according to known practice. The target in this example comprises four periodic structures (e.g., gratings) 32 to 35 positioned closely together so that they will all be within a measurement spot 31 formed by the metrology radiation illumination beam of the inspection apparatus. The four periodic structures thus are all simultaneously illuminated and simultaneously imaged on sensors 19 and 23. In an example dedicated to measurement of overlay, periodic structures 32 to 35 are themselves composite periodic structures formed by overlying periodic structures that are patterned in different layers of, e.g., the semi-conductor device formed on substrate W. Periodic structures 32 to 35 may have differently biased overlay offsets in order to facilitate measurement of overlay between the layers in which the different parts of the composite periodic structures are formed. The meaning of overlay bias will be explained below with reference to FIG. 8. Periodic structures 32 to 35 may also differ in their orientation, as shown, so as to diffract incoming radiation in X and Y directions. In one example, periodic structures 32 and 34 are X-direction periodic structures with bias offsets of +d, −d, respectively. Periodic structures 33 and 35 are Y-direction periodic structures with bias offsets +d, −d respectively. Separate images of these periodic structures can be identified in the image captured by sensor 23. This is only one example of a target. A target may comprise more or fewer than 4 periodic structures, or only a single periodic structure.

FIG. 9 shows an example of an image that may be formed on and detected by the sensor 23, using the target of FIG. 8 in the apparatus of FIG. 7, using the aperture plates 13NW or 13SE from FIG. 7D. While the pupil plane image sensor 19 cannot resolve the different individual periodic structures 32 to 35, the image sensor 23 can do so. The dark rectangle represents the field of the image on the sensor, within which the illuminated spot 31 on the substrate is imaged into a corresponding circular area 41. Within this, rectangular areas 42-45 represent the images of the small target periodic structures 32 to 35. If the targets are located in product areas, product features may also be visible in the periphery of this image field. Image processor and control system PU processes these images using pattern recognition to identify the separate images 42 to 45 of periodic structures 32 to 35. In this way, the images do not have to be aligned very precisely at a specific location within the sensor frame, which greatly improves throughput of the measuring apparatus as a whole.

Once the separate images of the periodic structures have been identified, the intensities of those individual images can be measured, e.g., by averaging or summing selected pixel intensity values within the identified areas. Intensities and/or other properties of the images can be compared with one another. These results can be combined to measure different parameters of the patterning process. Overlay performance is an important example of such a parameter.

FIG. 10 illustrates how, using for example the method described in PCT patent application publication no. WO 2011/012624 (incorporated herein in its entirety by reference), overlay error (i.e., undesired and unintentional overlay misalignment) between the two layers containing the component periodic structures 32 to 35 is measured. This measurement is done through identifying target asymmetry, as revealed by comparing the intensities in the +1^(st) order and −1 ^(st) order images of the target periodic structures (the intensities of other corresponding higher orders can be compared, e.g. +2^(nd) and −2^(nd) orders) to obtain a measure of the intensity asymmetry. At step S1, the substrate, for example a semiconductor wafer, is processed through a lithographic apparatus, such as the lithographic cell of FIG. 2, one or more times, to create a target including the periodic structures 32-35. At S2, using the inspection apparatus of FIG. 7, an image of the periodic structures 32 to 35 is obtained using only one of the first order diffracted beams (say −1). At step S3, whether by changing the illumination mode, or changing the imaging mode, or by rotating substrate W by 180° in the field of view of the inspection apparatus, a second image of the periodic structures using the other first order diffracted beam (+1) can be obtained. Consequently the +1 diffracted radiation is captured in the second image.

Note that, by including only half of the first order diffracted radiation in each image, the ‘images’ referred to here are not conventional dark field microscopy images. The individual target features of the target periodic structures will not be resolved. Each target periodic structure will be represented simply by an area of a certain intensity level. In step S4, a region of interest (ROI) is identified within the image of each component target periodic structure, from which intensity levels will be measured.

Having identified the ROI for each individual target periodic structure and measured its intensity, the asymmetry of the target, and hence overlay error, can then be determined. This is done (e.g., by the processor PU) in step S5 comparing the intensity values obtained for +1^(st) and −1 ^(st) orders for each target periodic structure 32-35 to identify their intensity asymmetry, e.g., any difference in their intensity. The term “difference” is not intended to refer only to subtraction. Differences may be calculated in ratio form. In step S6 the measured intensity asymmetries for a number of target periodic structures are used, together with knowledge of any known imposed overlay biases of those target periodic structures, to calculate one or more performance parameters of the patterning process in the vicinity of the target T.

FIGS. 11A-11D show schematic cross sections of target periodic structures (overlay periodic structures), with different bias offsets. These can be used as the target T on substrate W, as seen in FIGS. 7-9. Periodic structures with periodicity in the X direction are shown for the sake of example only. Different combinations of these periodic structures with different biases and with different orientations can be provided separately or as part of a target.

Starting with FIG. 11A, a target 600 formed in at least two layers, labeled L1 and L2, is shown. In the lower or bottom layer L1, a first periodic structure (the lower or bottom periodic structure), for example a grating, is formed by features 602 and spaces 604 on a substrate 606. In layer L2, a second periodic structure, for example a grating, is formed by features 608 and spaces 610. (The cross-section is drawn such that the features 602, 608 (e.g., lines) extend into the page.) The periodic structure pattern repeats with a pitch P in both layers. Features 602 and 608 may take the form of lines, dots, blocks and via holes. In the situation shown at FIG. 11A, there is no overlay contribution due to misalignment, e.g., no overlay error and no imposed bias, so that each feature 608 of the second structure lies exactly over a feature 602 in the first structure.

At FIG. 11B, the same target with a first known imposed bias +d is shown, such that the features 608 of the first structure are shifted by a distance d to the right, relative to the features of the second structure. The bias distance d might be a few nanometers in practice, for example 10 nm-20 nm, while the pitch P is for example in the range 300-1000 nm, for example 500 nm or 600 nm. At FIG. 11C, another feature with a second known imposed bias −d, such that the features of 608 are shifted to the left, is depicted. The value of d need not be the same for each structure. Biased periodic structures of this type shown at FIGS. 11A to 11C are described in the prior patent application publications mentioned above.

FIG. 11D shows schematically a phenomenon of structural asymmetry, in this case structural asymmetry in the first structure (bottom structure asymmetry). The features in the periodic structures at FIGS. 11A to 11C, are shown as perfectly square-sided, when a real feature would have some slope on the side, and a certain roughness. Nevertheless they are intended to be at least symmetrical in profile. The features 602 and/or spaces 604 at FIG. 11D in the first structure no longer have a symmetrical form at all, but rather have become distorted by one or more processing steps. Thus, for example, a bottom surface of each space has become tilted (bottom wall tilt). For example, side wall angles of the features and spaces have become asymmetrical. As a result of this, the overall target asymmetry of a target will comprise an overlay contribution independent of structural asymmetry (i.e., an overlay contribution due to misalignment of the first structure and second structure; itself comprised of overlay error and any known imposed bias) and a structural contribution due to this structural asymmetry in the target.

FIG. 12 is a schematic cross-section of an overlay periodic structure. As illustrated in FIG. 2, in an embodiment, the overlay periodic structure comprises two gratings. The first grating is shown on the left hand side of FIG. 12. The second grating is shown in the right hand side of FIG. 12.

The downward arrows in FIG. 12 represent the illuminating radiation incident on the gratings. The upward diagonal arrows represent the reflected +1 diffraction order radiation and the reflected −1 diffraction order radiation.

The intensity asymmetry A can be measured. The intensity asymmetry is the difference in intensity between the +1 and −1 diffraction orders. The intensity asymmetry A is proportional to overlay OV. In particular, the intensity asymmetry A is equal to the product of the overlay OV and the overlay sensitivity K.

By measuring the asymmetry A on two different targets with two different imposed additional shifts, the overlay sensitivity K can be determined, and the overlay OV can be measured. In FIG. 12, the additional shift is shown as d.

When overlay is measured by the method of FIG. 10 using only two biased periodic structures, the process-induced structural asymmetry cannot be distinguished from the overlay contribution due to misalignment, and overlay measurements (in particular to measure the undesired overlay error) become unreliable as a result. Structural asymmetry in the first structure (bottom periodic structure) of a target is a common form of structural asymmetry. It may originate, for example, in the substrate processing steps such as chemical-mechanical polishing (CMP), performed after the first structure was originally formed.

In PCT patent application publication no. WO 2013-143814, it is proposed to use three or more component periodic structures to measure overlay by a modified version of the method of FIG. 10. Three or more periodic structures of the type shown in FIGS. 11A to 11C are used to obtain overlay measurements that are to some extent corrected for structural asymmetry in the target periodic structures, such as is caused by bottom structure asymmetry in a practical patterning process. However, this method requires a new target design (e.g. different to that illustrated in FIG. 8) and therefore a new patterning device or patterning device pattern will be required. Furthermore, the target area is larger and therefore consumes more substrate area. In addition, the phase element of the overlay contribution resultant from structural asymmetry is ignored in this and other prior methods, meaning that the corrections are not as accurate as they could be if the phase element was also corrected for.

In FIG. 13 a curve 702 illustrates the relationship between overlay OV and intensity asymmetry A for an ‘ideal’ target having zero offset and no structural asymmetry within the individual periodic structures forming the target, and in particular within the individual periodic structure of the first structure. Consequently, the target asymmetry of this ideal target comprises only an overlay contribution due to misalignment of the first structure and second structure resultant from a known imposed bias and overlay error OV_(E). This graph, and the graph of FIG. 14, illustrate the principles behind the disclosure only, and in each graph, the units of intensity asymmetry A and overlay OV are arbitrary. Examples of actual dimensions will be given further below.

In the ‘ideal’ situation of FIG. 13, the curve 702 indicates that the intensity asymmetry A has a non-linear periodic relationship (e.g., sinusoidal relationship) with the overlay. The period P of the sinusoidal variation corresponds to the period or pitch P of the periodic structures, converted of course to an appropriate scale. The sinusoidal form is pure in this example, but can include harmonics in real circumstances.

As mentioned above, biased periodic structures (having a known imposed overlay bias) can be used to measure overlay, rather than relying on a single measurement. This bias has a known value defined in the patterning device (e.g. a reticle) from which it was made, that serves as an on-substrate calibration of the overlay corresponding to the measured intensity asymmetry. In the drawing, the calculation is illustrated graphically. In steps S1-S5, intensity asymmetry measurements A_(+d) and A_(−d) are obtained for periodic structures having imposed biases +d and −d respectively (as shown in FIG. 11B and FIG. 11C, for example). Fitting these measurements to the sinusoidal curve gives points 704 and 706 as shown. Knowing the biases, the true overlay error OV_(E) can be calculated. The pitch P of the sinusoidal curve is known from the design of the target. The vertical scale of the curve 702 is not known to start with, but is an unknown factor which can be referred to as a 1st harmonic proportionality constant, K. Thus, overlay sensitivity K is a measure of the sensitivity of the intensity asymmetry measurements to overlay. In an embodiment, it is a proportionality of the measured intensity with respect to overlay. It thus helps detect process dependency of overlay.

In equation terms, the relationship between overlay error OV_(E) and intensity asymmetry A is assumed to be: A _(±d) =K sin(OV _(E) ±d)  (1) where overlay error OV_(E) is expressed on a scale such that the target pitch P corresponds to an angle 2π radians. Using two measurements of gratings with different, known biases (e.g. +d and −d), the overlay error OV_(E) can be calculated using:

$\begin{matrix} {{OV}_{E} = {{atan}\left( {\frac{A_{+ d} + A_{- d}}{A_{+ d} - A_{- d}} \cdot {\tan(d)}} \right)}} & (2) \end{matrix}$

FIG. 14 shows a first effect of introducing structural asymmetry, for example the bottom periodic structure asymmetry illustrated in FIG. 11D. The ‘ideal’ sinusoidal curve 702 no longer applies. However, at least approximately, bottom periodic structure asymmetry or other structural asymmetry has the effect of adding an intensity shift term K₀ and a phase shift term ϕ to the intensity asymmetry A_(±d). The resulting curve is shown as 712 in the diagram, with label K₀ indicating the intensity shift term, and label ϕ indicating the phase offset term. The intensity shift term K₀ and phase shift term ϕ are dependent upon a combination of the target and a selected characteristic of the measurement radiation, such as the wavelength and/or polarization of the measurement radiation, and is sensitive to process variations. In equation terms, the relationship used for calculation in step S6 becomes: A _(±d) =K ₀ +K sin(OV _(E) ±d+ϕ)  (3) Where there is structural asymmetry, the overlay model described by equation (2) will provide overlay error values which are impacted by the intensity shift term K₀ and phase shift term ϕ, and will be inaccurate as a consequence. The structural asymmetry will also result in differences in measurements of the same target using one or more different measurement parameters (e.g., wavelength of the measurement beam, polarization of the measurement beam, etc.), when mapping the overlay error, because intensity and phase shift are, e.g., wavelength and/or polarization dependent.

The overlay calculations of modified step S6 rely on certain assumptions. Firstly, it is assumed intensity asymmetry behaves as a sine function of the overlay, with the period P corresponding to the grating pitch. These assumptions are valid for present overlay ranges. The number of harmonics can be designed to be small, because the small pitch-wavelength ratio only allows for a small number of propagating diffraction orders from the grating. However, in practice the overlay contribution to the intensity asymmetry due to misalignment may not necessarily be truly sinusoidal, and may not necessarily be completely symmetrical about OV=0.

So, the effect of structural asymmetry can be generally formulated as: ΔI ₊ =K(O+d)+ΔI _(B)  (4) Δ⁻ =K(O−d)+ΔI _(B)  (5) where ΔI. (also synonymous with A−) and ΔI₊ (also synonymous with A₊) represent the intensity asymmetry measured and ΔI_(BG) is the contribution to the intensity asymmetry of the structural asymmetry. And so, the overlay error ΔOV can be considered as a function of ΔI_(BG)/K.

Now, it has been further discovered that, in addition to or alternatively to structural asymmetry in a target, a stack difference between adjacent periodic structures of a target or between adjacent targets may be a factor that adversely affects the accuracy of measurement, such as overlay measurement. Stack difference may be understood as an un-designed difference in physical configurations between adjacent periodic structures or targets. Stack difference causes a difference in an optical property (e.g., intensity, polarization, etc.) of measurement radiation between the adjacent periodic structures or targets that is due to other than overlay error, other than intentional bias and other than structural asymmetry common to the adjacent periodic structures or targets. Stack difference includes, but is not limited to, a thickness difference between the adjacent periodic structures or targets (e.g., a difference in thickness of one or more layers such that one periodic structure or target is higher or lower than another periodic structure or target designed to be at a substantially equal level), a refractive index difference between the adjacent periodic structures or targets (e.g., a difference in refractive index of one or more layers such that the combined refractive index for the one or more layers for one periodic structure or target is different than the combined refractive index for the one or more layers for of another periodic structure or target even though designed to have a substantially equal combined refractive index), a difference in material between the adjacent periodic structures or targets (e.g., a difference in the material type, material uniformity, etc. of one or more layers such that there is a difference in material for one periodic structure or target from another periodic structure or target designed to have a substantially same material), a difference in the grating period of the structures of adjacent periodic structures or targets (e.g., a difference in the grating period for one periodic structure or target from another periodic structure or target designed to have a substantially same grating period), a difference in depth of the structures of adjacent periodic structures or targets (e.g., a difference due to etching in the depth of structures of one periodic structure or target from another periodic structure or target designed to have a substantially same depth), a difference in width (CD) of the features of adjacent periodic structures or targets (e.g., a difference in the width of features of one periodic structure or target from another periodic structure or target designed to have a substantially same width of features), etc. In some examples, the stack difference is introduced by processing steps, such as CMP, layer deposition, etching, etc. in the patterning process. In an embodiment, periodic structures or targets are adjacent if within 200 μm of each other, within 150 μm of each other, within 100 μm of each other, within 75 μm of each other, within 50 μm of each other, within 40 μm of each other, within 30 μm of each other, within 20 μm of each other, or within 10 μm of each other.

The effect of stack difference (which can be referred to as grating imbalance between gratings) can be generally formulated as: ΔI ₊=(K+ΔK)(OV+d)  (6) ΔI ⁻=(K−ΔK)(OV−d)  (7) wherein ΔK represents a difference in the overlay sensitivity attributable to the stack difference. And so, the overlay error ΔOV can be proportional to

$\frac{\Delta\; K}{K}{d.}$

So, in order to characterize the stack difference, one or more stack difference parameters can be defined. As noted above, a stack difference parameter is a measure of the un-designed different physical configuration of the adjacent periodic structures or targets. In an embodiment, the stack difference parameter can be determined from evaluating cross-sections of the adjacent periodic structures or targets.

In an embodiment, the stack difference parameter can be determined for lower adjacent gratings of a composite grating by evaluating the lower adjacent gratings before the upper gratings are applied. In an embodiment, the stack difference parameter can be derived from a reconstruction of the adjacent periodic structures or targets from optical measurements of the adjacent periodic structures or targets or from cross-sections of the adjacent periodic structures or targets. That is, the physical dimensions, characteristics, materials properties, etc. are reconstructed and the differences between the adjacent periodic structures or targets are determined to arrive at a stack difference parameter.

An embodiment of the stack difference parameter is a periodic structure intensity imbalance (GI) which can be defined as:

$\begin{matrix} {{GI} = {2*\frac{{\hat{I}}^{+ d} - {\hat{I}}^{- d}}{{\hat{I}}^{+ d} + {\hat{I}}^{- d}}}} & (8) \end{matrix}$ where Î^(+d) is the average of the +1^(st) diffraction order intensity signal diffracted by a first periodic structure having a +d bias, I₊₁ ^(+d), and −1^(st) diffraction order intensity signal diffracted by the first periodic structure having the +d bias, I⁻¹ ^(+d). Similarly, Î^(−d) is the average of the +1^(st) diffraction order intensity signal diffracted by a second periodic structure having a −d bias, I₊₁ ^(d), and −1 ^(st) diffraction order intensity signal diffracted by the second periodic structure having the −d bias, I⁻¹ ^(−d). In an embodiment, the periodic structure intensity imbalance (GI) can be a derived version, such as

$\frac{{\hat{I}}^{+ d} - {\hat{I}}^{- d}}{{\hat{I}}^{+ d} + {\hat{I}}^{- d}},\frac{{\hat{I}}^{+ d} + {\hat{I}}^{- d}}{{\hat{I}}^{+ d} - {\hat{I}}^{- d}},$ etc.

Now, in the face of structural asymmetry, stack difference and any other process variabilities, it would be desirable to derive a combination of target layout, measurement beam wavelength, measurement beam polarization, etc. that would yield an accurate measurement of the desired process parameter (e.g., overlay) and/or that yields measurement values of the desired process parameter that is robust to process variability. Thus, it would be desirable, for example, to arrive at a desirably optimum selection of the target-measurement parameter combination so as to obtain more accurate process parameter measurement and/or that yields measurement values of the desired process parameter that is robust to process variability.

The measurement accuracy and/or sensitivity of the target may vary with respect to one or more attributes of the target itself and/or one or more attributes of the measurement radiation provided onto the target, for example, the wavelength of the radiation, the polarization of the radiation, and/or the intensity distribution (i.e., angular or spatial intensity distribution) of the radiation. In an embodiment, the wavelength range of the radiation is limited to one or more wavelengths selected from a range (e.g., selected from the range of about 400 nm to 900 nm). Further, a selection of different polarizations of the radiation beam may be provided and various illumination shapes can be provided using, for example, a plurality of different apertures.

So, to enable such selection and measurement, a substrate measurement recipe can be used that specifies one or more parameters of the measurement using the measurement system. In an embodiment, the term “substrate measurement recipe” includes one or more parameters of the measurement itself, one or more parameters of a pattern measured, or both.

In this context, a pattern measured (also referred to as a “target” or “target structure”) may be a pattern that is optically measured, e.g., whose diffraction is measured. The pattern measured may be a pattern specially designed or selected for measurement purposes. Multiple copies of a target may be placed on many places on a substrate. For example, a substrate measurement recipe may be used to measure overlay. In an embodiment, a substrate measurement recipe may be used to measure another process parameter (e.g., dose, focus, CD, etc.) In an embodiment, a substrate measurement recipe may be used for measuring alignment of a layer of a pattern being imaged against an existing pattern on a substrate; for example, a substrate measurement recipe may be used to align the patterning device to the substrate, by measuring a relative position of the substrate.

In an embodiment, if the substrate measurement recipe comprises one or more parameters of the measurement itself, the one or more parameters of the measurement itself can include one or more parameters relating to a measurement beam and/or measurement apparatus used to make the measurement. For example, if the measurement used in a substrate measurement recipe is a diffraction-based optical measurement, one or more parameters of the measurement itself may include a wavelength of measurement radiation, and/or a polarization of measurement radiation, and/or measurement radiation intensity distribution, and/or an illumination angle (e.g., incident angle, azimuth angle, etc.) relative to the substrate of measurement radiation, and/or the relative orientation relative to a pattern on the substrate of diffracted measurement radiation, and/or number of measured points or instances of the target, and/or the locations of instances of the target measured on the substrate. The one or more parameters of the measurement itself may include one or more parameters of the metrology apparatus used in the measurement, which can include detector sensitivity, numerical aperture, etc.

In an embodiment, if the substrate measurement recipe comprises one or more parameters of a pattern measured, the one or more parameters of the pattern measured may include one or more geometric characteristics (such as a shape of at least part of the pattern, and/or orientation of at least part of the pattern, and/or a pitch of at least part of the pattern (e.g., pitch of a periodic structure including the pitch of an upper periodic structure in a layer above that of a lower periodic structure and/or the pitch of the lower periodic structure), and/or a size (e.g., CD) of at least part of the pattern (e.g., the CD of a feature of a periodic structure, including that of a feature of the upper periodic structure and/or the lower periodic structure), and/or a segmentation of a feature of the pattern (e.g., a division of a feature of a periodic structure into sub-structures), and/or a length of a periodic structure or of a feature of the periodic structure), and/or a materials property (e.g., refractive index, extinction coefficient, material type, etc.) of at least part of the pattern, and/or an identification of the pattern (e.g., distinguishing a pattern being from another pattern), etc.

A substrate measurement recipe may be expressed in a form like (r₁, r₂, r₃, . . . r_(n); t₁, t₂, t₃ . . . t_(m)), where r_(i) are one or more parameters of the measurement and t_(j) are one or more parameters of one or more patterns measured. As will be appreciated, n and m can be 1. Further, the substrate measurement recipe does not need to have both one or more parameters of the measurement and one or more parameters of one or more patterns measured; it can have just one or more parameters of the measurement or have just one or more parameters of one or more patterns measured.

A target may be subjected to measurement using two substrate measurement recipes A and B, e.g., differ on the stage at which a target is measured (e.g., A measures a target when it comprises a latent image structure and B measures a target when it doesn't comprise a latent image structure) and/or differ on the parameters of their measurement. Substrate measurement recipes A and B can at least differ on the target measured (e.g., A measures a first target and B measures a second different target). Substrate measurement recipes A and B may differ on the parameters of their measurement and target measurement. Substrate measurement recipes A and B may not even be based on the same measurement technique. For example recipe A may be based on diffraction-based measurement and recipe B may be based on scanning electron microscope (SEM) or atomic force microscopy (AFM) measurement.

Accordingly, in an embodiment, to determine one or more substrate measurement recipes that would yield an accurate measurement of the desired process parameter (e.g., overlay) and/or that yields measurement values of the desired process parameter that is robust to process variability, a plurality of substrate measurement recipes can be evaluated against one or more performance indicators to identify such one or more accurate and/or robust substrate measurement recipes.

An embodiment of a method to determine one or more substrate measurement recipes that would yield an accurate measurement of the desired process parameter (e.g., overlay) and/or that yields measurement values of the desired process parameter that is robust to process variability is schematically depicted. In this example method, a plurality of different metrology targets are each evaluated against a plurality of different values of measurement beam wavelength (i.e., the wavelengths available from an inspection apparatus that would be used to measure the target) and a plurality of different values of polarization (i.e., the polarizations available from the inspection apparatus that would be used to measure the target); each particular combination of target, wavelength and polarization corresponding to a particular substrate measurement recipe. However, the method is not so limited. For example, the method can be used to evaluate other or additional parameters than wavelength and polarization. As another example, the method can be used to evaluate just a single target (e.g., against a plurality of different wavelengths and polarizations, against a plurality of different wavelengths for a single polarization, against a single wavelength for a plurality of different polarizations, etc.). As another example, the method can be used to evaluate a plurality of targets against a plurality of different wavelengths for a single polarization. As another example, the method can be used to evaluate a plurality of targets against a plurality of different polarizations for a single wavelength.

Further, while various steps are shown in sequence, they need not necessarily be performed in that sequence. Further, all steps need not be performed. For example, one or more of the step may be performed. So, any combination selected from the steps can be performed.

At 1400, a first analysis of data for a single target against a plurality of different wavelengths and for a plurality of different polarizations (in this case, two polarizations) is performed. The data can be obtained experimentally or obtained from production measurements using the target. For example, a plurality of instances of a target under consideration can be printed across a substrate using the patterning process for which the target will be used and then each instance measured with the applicable inspection apparatus at a plurality of different settings (e.g., different wavelengths, different polarizations, etc.).

A process parameter (e.g., overlay, alignment, focus) measurement resulting from using a substrate measurement recipe to measure a target may be simulated. In the simulation, one or more parameters of the measurement are determined using (e.g., provided by, or determined from) the parameters r_(i) and/or t_(j) of the substrate measurement recipe. For example, the interaction between the radiation and the target corresponding to the substrate measurement recipe can be determined from those parameters of the substrate measurement recipe by using, for example, a Maxwell solver and rigorous coupled-wave analysis (RCWA) or by other mathematical modelling. So, the measurement expected using the target and the associated substrate measurement recipe can be determined from the interaction. So, in certain circumstances, for example to determine targets that yield strong signals, the data can be obtained using a simulator of the measurement process; the simulator can mathematically derive how a particular target of particular characteristics (e.g., a target specified in terms of pitch, feature width, material types, etc.) would be measured using an inspection apparatus according to the measurement technique (e.g., diffraction-based overlay measurement) of the inspection apparatus by, for example, calculating the intensity that would be measured in a detector of, e.g., the apparatus of FIG. 7. To obtain robustness data, the simulator can introduce a perturbation within a certain range (e.g., an up to 10% change, an up to 5% change, an up to 2% change, an up to 1% change, or an up to 0.5% change) to mimic process variation (which can be extended across a substrate).

So, the experimental method or simulation can yield values for particular parameters or indicators such as OV, K, etc. using, for example, the formulas described above.

One such indicator is stack sensitivity (SS) (also consider as signal contrast). Stack sensitivity can be understood as a measure of how much the intensity of the signal changes as overlay changes because of diffraction between target (e.g., grating) layers. That is, in an overlay context, it detects the contrast between upper and lower periodic structure of an overlay target and thus represents a balance between diffraction efficiencies between the upper and lower periodic structure. It is thus an example measure of sensitivity of the measurement. In an embodiment, stack sensitivity is the ratio between intensity asymmetry and average intensity. In an embodiment, stack sensitivity can be formulated as SS=KL/IM, wherein L is a user defined constant (e.g., in an embodiment, the value L is 20 nm and/or the value of the bias d) and IM is the mean intensity of the measurement beam diffracted by the target. In an embodiment, the stack sensitivity for a substrate measurement recipe should be maximized. However, it has been discovered that use of a substrate measurement recipe with maximum stack sensitivity may not be best. For example, a measurement beam wavelength for which stack sensitivity is maximum may correspond to low overlay sensitivity and poor process robustness.

Examples of substrate measurement recipe data are presented in FIGS. 15 and 16. The data can represent a dependency of measurement data as a function of one or more substrate measurement recipe parameters, in particular one or more parameters of the measurement itself. FIG. 15 is a graph of diffraction efficiency for a target for measurement at various polarization angles for a single wavelength. Diffraction efficiency is directly proportional to the mean intensity of the measurement beam diffracted by the target.

In FIG. 15, the X axis represents the polarization angle θ (in degrees) of radiation that illuminates the metrology target in the measurement of the metrology target. The polarization angle θ is the angle of the electric field vector of the illuminating light with respect to p polarization. The Y axis represents the polarization angle φ (in degrees) of a polarizer of a detector that detects radiation that reflects from the metrology target. The polarization angle φ is defined as the angle of the electric field vector of the illuminating light with respect to s polarization direction. The scale on the right represents the diffraction efficiency scale. As shown in FIG. 15, the diffraction efficiency varies depending on the polarization angles. In FIGS. 15 and 16, it is assumed that the angle between the p polarization and the s polarization is 30°.

In an embodiment a method of the present invention comprises evaluating a plurality of polarization characteristics associated with measurement of a metrology target of a substrate processed using a patterning process, against one more measurement quality parameters. FIG. 15 is an example of the results of such an evaluation. In the example of FIG. 15, the polarization characteristics that are evaluated are the polarization angles of the illuminating radiation and the polarizer of the detector. However, other polarization characteristics can be evaluated. For example, in an embodiment the polarization characteristics comprise the retardance i.e. the phase differences between orthogonal polarization components and/or the wavelength of radiation used. The orthogonal polarization components are p polarization and s polarization.

In the example of FIG. 15, the measurement quality parameter is diffraction efficiency. However, the polarization characteristics can be evaluated against other measurement quality parameters. For example, other possible measurement quality parameters include stack sensitivity (e.g. see FIG. 16) and overlay sensitivity (e.g. see FIG. 18).

FIG. 16 is a graph of stack sensitivity for a target for measurement at various polarizations for a single wavelength. In FIG. 16, the X axis and the Y axis are the same as in FIG. 15. In the example of FIG. 15, the measurement quality parameter is stack sensitivity. The scale on the right is the stack sensitivity scale. As shown in FIG. 16, the stack sensitivity varies depending on the polarization angles of the illumination radiation and the polarizer of the detector.

From the data shown in FIG. 15 and FIG. 16, desirable polarization angles for metrology measurements can be selected. In an embodiment, the method comprises selecting one or more polarization characteristics from the plurality of polarization characteristics based on one or more of the measurement quality parameters.

For example, from the data shown in FIG. 16, the maximum stack sensitivity that can be achieved for pure p polarization is a stack sensitivity of 0.08. However, the stack sensitivity can be greater than 0.08 by varying the polarization angle of the illumination radiation and the polarizer of the detector. By controlling the polarization angles, the achievable stack sensitivity can be significantly increased. As shown in FIG. 16, the maximum stack sensitivity that can be achieved is 0.2, which is an improvement of 2.5× on the maximum stack sensitivity that is achievable for pure p polarization.

In an embodiment, the selecting step comprises selecting one or more polarization characteristics that result in one or more of the measurement quality parameters being within a certain finite range from a maximum or minimum value from among the evaluated plurality of polarization characteristics. For example, from the data shown in FIGS. 15 and 16, polarization angles that result in a maximum diffraction efficiency and/or a maximum stack sensitivity value could be chosen.

It may not be necessary or desirable for the polarization angles that achieve the absolute maximum diffraction efficiency and/or stack sensitivity values to be selected. For example, from FIGS. 15 and 16 it can be seen that the maximum stack sensitivity values may correspond to polarization angles that result in particularly low diffraction efficiency values. The decision about which measurement quality parameters should be maximised (or should be near maximum) can be made depending on what qualities of the measurement are desirable. It may be necessary to perform a trade-off between different desirable qualities.

In metrology measurements, there are different qualities that can be important. One such quality is the detectability. The detectability is closely related to the single to noise ratio of the metrology measurement. The detectability for a metrology measurement depends on the diffraction efficiency of the metrology target and the stack sensitivity. The diffraction efficiency is related to the amount of radiation that reflects from the metrology target. The stack sensitivity is related to the relative change in the amount of radiation that is reflected by the metrology target per nm of overlay error. Hence, a greater diffraction efficiency leads to metrology measurements with a greater detectability. Similarly, a greater stack sensitivity results in metrology measurements that have a greater detectability.

Another desirable quality for metrology measurements is the accuracy of the metrology measurements. Additionally, robustness is also desirable. In particular, it is desirable for the metrology measurements to be relatively insensitive to any undesirable structural asymmetry in the bottom grating of the metrology target. Metrology measurements that are insensitive to structural feature asymmetry in the bottom grating of the metrology are considered to be robust. The accuracy and robustness depend on the overlay sensitivity. A greater overlay sensitivity results in metrology measurements that have greater accuracy and robustness.

According the present invention, qualities such as detectability and accuracy can be optimised by optimising the polarization characteristics of the measurements.

In an embodiment, the selecting step comprises selecting one or more polarization characteristics that result in one or more of the measurement quality parameters reaching a threshold value. In an embodiment, candidate polarization characteristics are ruled out if they result in one or more of the measurement quality parameters not reaching the threshold value. For example, it may be decided that polarization angles that result in a diffraction efficiency of below 0.1 are discarded. This leaves a subset of possible polarization angles that can be chosen for the metrology measurements.

In an embodiment, the subset of candidate polarization angles can then be narrowed further by considering a different measurement quality parameter. For example, the polarization angles that achieve a maximum (or near maximum) stack sensitivity from within the subset of the polarization angles that achieve a diffraction efficiency of 0.1 may be selected. The selection steps may be performed in the opposite order. For example, first the candidate polarization angles can be narrowed down by selecting a subset of polarization angles that result in a stack sensitivity within e.g. 0.04 of the maximum stack sensitivity. This subset of candidate polarization angles can then be narrowed further by selecting only the polarization angles that achieve a diffraction efficiency of at least 0.05. These are merely examples of how the polarization characteristics can be selected based on the evaluated data in order to improve the detectability and accuracy/robustness of the metrology measurements.

An embodiment of the invention is expected to achieve an improvement in the detectability of metrology measurements. This may in particular be achieved by selecting polarization characteristics that results in higher diffraction efficiency and/or stack sensitivity values and/or overlay sensitivity values.

An embodiment of the invention is expected to achieve an increase in the accuracy and/or robustness of metrology measurements. This may be in particular be achieved by selecting polarization characteristics that results in a higher overlay sensitivity.

As can be seen from FIGS. 15 and 16, the possible range of stack sensitivity values and diffraction efficiency values is large. In some cases the stack sensitivity can even be made to be positive or negative, depending on the exact polarization angles. Similarly, the overlay sensitivity can be made to be positive or negative, depending on the exact polarization angles. The stack sensitivity, diffraction efficiency and overlay sensitivity can vary greatly depending on the phase differences between orthogonal polarization components.

An embodiment of the invention is expected to reduce the impact of mark deformations such as feature asymmetry in the bottom grating of the metrology targets. This can be done in particular by selecting polarization characteristics that result in larger overlay sensitivity values. An embodiment of the invention is expected to speed up metrology measurements that are robust with respect to mark deformations. In particular, the impact of mark deformations has previously been reduced by using two different wavelengths of radiation. In an embodiment, two different polarization characteristics are used instead of two different wavelengths. A polarization switch may be faster than a wavelength switch, thereby speeding up the measurement process. Furthermore, a polarization switch does not suffer from chromatic differences in the optics between the two settings.

In an embodiment, two different polarization settings can be combined in a single measurement. For example, the overlay error can be measured by using two different sets of polarization angles (and/or two different sets of phase differences). In particular, by combining polarization characteristics that provide a positive overlay sensitivity with polarization characteristics that result in a negative overlay sensitivity, the overlay error can be measured more accurately for metrology targets that have undesirable structural asymmetry in the bottom grating.

In an embodiment the evaluating step is performed by simulation. For example, the data shown in FIGS. 15 and 16 was obtained by simulation.

FIG. 17 is a diagram of the path of radiation through part of a metrology apparatus according to an embodiment of the invention. In FIG. 17, the direction from left to right represents that downstream direction of radiation used in the metrology measurements. In an embodiment the metrology apparatus comprises an illumination polarizer 171. The illumination polarizer 171 is configured to control the polarization angle θ of the radiation that illuminates the substrate W. In an embodiment the illumination polarizer 171 is a rotatable polarizer.

In an embodiment the metrology apparatus comprises an illumination retarder 172. The illumination retarder is positioned between the illumination polarizer 171 and the substrate W. The illumination retarder 172 is configured to control the phase difference (or ellipticity) between the s polarization component and the p polarization component of the illumination radiation. In an embodiment the illumination retarder 172 is a rotatable retarder.

After the radiation has illuminated the substrate W, reflected radiation is detected by a detector. As depicted in FIG. 17, in an embodiment the detector comprises a detection retarder 173. The detection retarder 173 is configured to impart a phase difference between the orthogonal polarization components of the radiation reflecting from the substrate W. In an embodiment the detector comprises a detection polarizer 174. The detection polarizer 174 is configured to control a polarization of radiation that is detected at the detector. Besides a mechanical rotation of fixed elements, also electro-optical elements (or e.g. switchable liquid crystal cells) may be used to obtain the desired polarizing behavior for the polarizers and/or the retarders.

It is not essential for the retarders to be provided. In an embodiment the metrology apparatus comprises the illumination polarizer 171 and/or the detection polarizer 174, without comprising the retarders. Furthermore, the polarizers are not essential. In an embodiment, polarizers and retarders are provided only in the illumination branch. In an embodiment, a pixelated light modulator is provided to manipulate the polarization of the radiation in a pupil plane. This allows the optimization described above to be performed substantially per pupil point instead of with one generic setting for all pupil points.

The retarders are configured to adjust the phase differences in the illumination and detection branches of the metrology apparatus. By tuning the phase differences between the s and p components. The parameters such as overlay sensitivity can be controlled. FIG. 18 is a chart showing how the overlay sensitivity varies depending on the phase difference ΔE (Y axis) and the wavelength (X axis) of the illumination radiation. In FIG. 18, the phase difference ΔE is the phase difference (in radians) between the s and p components of the illumination radiation. The scale on the right represents the overlay sensitivity scale.

As shown in FIG. 18, the overlay sensitivity is dependent on the phase difference ΔE. In FIG. 19 it is shown how the overlay sensitivity is physically increased. The overlay sensitivity depends on the strength of the amplitudes reflected from the bottom and top gratings. Assuming the detector polarization is set so as to detect only the s component, the S component at the detector will arise from the illumination light s to s unconverted part and the illuminating light p to s converted part which will coherently add up at the detector. By tuning the retardance at the illumination part, the coherent addition can be strengthened. Thus the overlay sensitivity can be increased. Unpolarized illumination radiation has fast random phase differences. The effects of the fast random phase differences cancel each other out and in effect both polarizations add up incoherently. However, polarized illumination radiation has a phase difference ΔE that can be controlled. The phase difference ΔE can be tuned to maximize overlay sensitivity, for example. The phase difference ΔE is related to the ellipticity.

In an embodiment, the method is combined with target design optimization. In particular, in an embodiment the method comprises selecting a pitch and optionally other features of the metrology target so as to maximise one or more of the measurement quality parameters. In an embodiment, the shape of sub-segmentations of the target grating lines is selected to modify and optimise the polarizing properties of each of the gratings.

An embodiment of the invention may take the form of a computer program containing one or more sequences of machine-readable instructions describing a method as disclosed herein, or a data storage medium (e.g. semiconductor memory, magnetic or optical disk) having such a computer program stored therein. Further, the machine readable instruction may be embodied in two or more computer programs. The two or more computer programs may be stored on one or more different memories and/or data storage media.

One or more aspects disclosed herein may be implemented in a control system. Any control system described herein may each or in combination be operable when the one or more computer programs are read by one or more computer processors located within at least one component of an apparatus. The control systems may each or in combination have any suitable configuration for receiving, processing, and sending signals. One or more processors are configured to communicate with the at least one of the control systems. For example, each control system may include one or more processors for executing the computer programs that include machine-readable instructions for the methods described above. The control systems may include data storage medium for storing such computer programs, and/or hardware to receive such medium. So the control system(s) may operate according the machine readable instructions of one or more computer programs.

Further embodiments according to the invention are further described in below numbered clauses:

1. A method comprising:

-   -   evaluating a plurality of polarization characteristics         associated with measurement of a metrology target of a substrate         processed using a patterning process, against one or more         measurement quality parameters; and     -   selecting one or more polarization characteristics from the         plurality of polarization characteristics based on one or more         of the measurement quality parameters.         2. The method of clause 1, wherein the polarization         characteristics comprise polarization angles and/or phase         differences between orthogonal polarization components.         3. The method of clause 2, wherein the polarization angles         comprises polarization angles of radiation that illuminates the         metrology target in the measurement of the metrology target.         4. The method of clause 2 or 3, wherein the polarization angles         comprises polarization angles of a polarizer of a detector that         detects radiation that reflects from the metrology target in the         measurement of the metrology target.         5. The method of any of clauses 2 to 4, wherein the polarization         characteristics comprise phase differences between orthogonal         polarizations of radiation that illuminates the metrology target         in the measurement of the metrology target.         6. The method of any of clauses 2 to 5, wherein the polarization         characteristics comprise phase differences imparted by a         retarder of a detector that detects radiation that reflects from         the metrology target in the measurement of the metrology target.         7. The method of any preceding clause, wherein the measurement         quality parameters comprise one or more of stack sensitivity,         overlay sensitivity and diffraction efficiency.         8. The method of any preceding clause, wherein the selecting         step comprises selecting one or more polarization         characteristics from the plurality of polarization         characteristics that result in one or more of the measurement         quality parameters being within a certain finite range from a         maximum or minimum value from among the evaluated plurality of         polarization characteristics.         9. The method of any preceding clause, wherein the selecting         step comprises selecting one or more polarization         characteristics from the plurality of polarization         characteristics that result in one or more of the measurement         quality parameters reaching a threshold value.         10. The method of clause 9, wherein the selecting step comprises         selecting one or more polarization characteristics from the         plurality of polarization characteristics that result in another         one or more of the measurement quality parameters being within a         certain finite range from a maximum or minimum value from among         the evaluated plurality of polarization characteristics that         result in said one or more of the measurement quality parameters         reaching a threshold value.         11. The method of any preceding clause, wherein the evaluating         step is performed by simulation.         12. A measurement method comprising measuring a metrology target         on a substrate according to the one or more polarization angles         selected in the method of any preceding clause.         13. A metrology apparatus for measuring a parameter of a         lithographic process, the metrology apparatus being operable to         perform the method of any preceding clause.         14. A non-transitory computer program product comprising         machine-readable instructions for causing a processor to cause         performance of the method of any of clauses 1-12.         15. A system comprising:     -   an inspection apparatus configured to provide a beam of         radiation on two adjacent periodic structures or measurement         targets on a substrate and to detect radiation diffracted by the         targets to determine a parameter of a patterning process; and     -   the non-transitory computer program product of clause 14.         16. The system of clause 15, further comprising a lithographic         apparatus comprising a     -   support structure configured to hold a patterning device to         modulate a radiation beam and a projection optical system         arranged to project the modulated radiation beam onto a         radiation-sensitive substrate.         17. A method to optimize a metrology measurement comprising:     -   the method of any of clauses 1-12; and     -   selecting a pitch and other features of the metrology target so         as to maximize one or more of the measurement quality         parameters.         Although specific reference may have been made above to the use         of embodiments in the context of optical lithography, it will be         appreciated that embodiments of the invention may be used in         other applications, for example imprint lithography, and where         the context allows, is not limited to optical lithography. In         imprint lithography, a topography in a patterning device defines         the pattern created on a substrate. The topography of the         patterning device may be pressed into a layer of resist supplied         to the substrate whereupon the resist is cured by applying         electromagnetic radiation, heat, pressure or a combination         thereof. The patterning device is moved out of the resist         leaving a pattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g., having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g., having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams or electron beams.

The term “lens”, where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components.

The foregoing description of the specific embodiments reveals the general nature of embodiments of the invention such that others can, by applying knowledge within the skill of the art, readily modify and/or adapt for various applications such specific embodiments, without undue experimentation, without departing from the general concept of the present invention. Therefore, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein. It is to be understood that the phraseology or terminology herein is for the purpose of description by example, and not of limitation, such that the terminology or phraseology of the present specification is to be interpreted by the skilled artisan in light of the teachings and guidance.

The breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents. 

The invention claimed is:
 1. A method comprising: evaluating a plurality of polarization characteristics against one or more measurement quality parameters, the plurality of polarization characteristics being associated with measurement of a metrology target of a substrate processed using a patterning process, wherein the polarization characteristics comprise polarization angles; and selecting one or more polarization characteristics from the plurality of polarization characteristics based on one or more of the measurement quality parameters.
 2. The method of claim 1, wherein the polarization characteristics further comprise phase differences between orthogonal polarization components.
 3. The method of claim 1, wherein the polarization angles comprise polarization angles of radiation that illuminate the metrology target in the measurement of the metrology target.
 4. The method of claim 1, wherein the polarization angles comprise polarization angles of a polarizer of a detector that detects radiation that reflects from the metrology target in the measurement of the metrology target.
 5. The method of claim 2, wherein the polarization characteristics comprise phase differences between orthogonal polarizations of radiation that illuminate the metrology target in the measurement of the metrology target.
 6. The method of claim 2, wherein the polarization characteristics comprise phase differences imparted by a retarder of a detector that detects radiation that reflects from the metrology target in the measurement of the metrology target.
 7. The method of claim 1, wherein the measurement quality parameters comprise one or more of stack sensitivity, overlay sensitivity, or diffraction efficiency.
 8. The method of claim 1, wherein the selecting comprises selecting one or more polarization characteristics from the plurality of polarization characteristics that result in one or more of the measurement quality parameters being within a certain finite range from a maximum or minimum value from among the evaluated plurality of polarization characteristics.
 9. The method of claim 2, wherein the selecting comprises selecting one or more polarization characteristics from the plurality of polarization characteristics that result in one or more of the measurement quality parameters reaching a threshold value.
 10. The method of claim 9, wherein the selecting comprises selecting one or more polarization characteristics from the plurality of polarization characteristics that result in another one or more of the measurement quality parameters being within a certain finite range from a maximum or minimum value from among the evaluated plurality of polarization characteristics that result in the one or more of the measurement quality parameters reaching a threshold value.
 11. The method of claim 1, wherein the evaluating is performed by simulation.
 12. A measurement method comprising measuring a metrology target on a substrate according to one or more polarization angles selected in a method comprising: evaluating a plurality of polarization characteristics against one or more measurement quality parameters, the plurality of polarization characteristics being associated with measurement of a metrology target of a substrate processed using a patterning process, wherein the polarization characteristics comprise polarization angles; and selecting one or more polarization characteristics from the plurality of polarization characteristics based on one or more of the measurement quality parameters.
 13. A metrology apparatus for measuring a parameter of a lithographic process, the metrology apparatus being operable to perform the method comprising: evaluating a plurality of polarization characteristics against one or more measurement quality parameters, the plurality of polarization characteristics being associated with measurement of a metrology target of a substrate processed using a patterning process, wherein the polarization characteristics comprise polarization angles; and selecting one or more polarization characteristics from the plurality of polarization characteristics based on one or more of the measurement quality parameters.
 14. A non-transitory computer program product comprising machine-readable instructions for causing a processor to cause performance of operations comprising: evaluating a plurality of polarization characteristics against one or more measurement quality parameters, the plurality of polarization characteristics being associated with measurement of a metrology target of a substrate processed using a patterning process, wherein the polarization characteristics comprise polarization angles; and selecting one or more polarization characteristics from the plurality of polarization characteristics based on one or more of the measurement quality parameters.
 15. A system comprising: an inspection apparatus configured to provide a beam of radiation on two adjacent periodic structures or measurement targets on a substrate and to detect radiation diffracted by the targets to determine a parameter of a patterning process; and a non-transitory computer program product comprising machine-readable instructions for causing a processor to cause performance of operations comprising: evaluating a plurality of polarization characteristics against one or more measurement quality parameters, the plurality of polarization characteristics being associated with measurement of a metrology target of a substrate processed using a patterning process, wherein the polarization characteristics comprise polarization angles; and selecting one or more polarization characteristics from the plurality of polarization characteristics based on one or more of the measurement quality parameters.
 16. The system of claim 15, further comprising a lithographic apparatus comprising a support structure configured to hold a patterning device to modulate a radiation beam and a projection optical system arranged to project the modulated radiation beam onto a radiation-sensitive substrate.
 17. A method to optimize a metrology measurement comprising: the method comprising: evaluating a plurality of polarization characteristics against one or more measurement quality parameters, the plurality of polarization characteristics being associated with measurement of a metrology target of a substrate processed using a patterning process, wherein the polarization characteristics comprise polarization angles; and selecting one or more polarization characteristics from the plurality of polarization characteristics based on one or more of the measurement quality parameters; and selecting a pitch and other features of the metrology target so as to maximize one or more of the measurement quality parameters. 